<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  
  <h2>当前计数: <span class="counter"></span></h2>
  <button class="add">+1</button>
  <button class="sub">-1</button>

  <script>
    // 1.获取dom
    const h2El = document.querySelector("h2")
    const counterEl = document.querySelector(".counter")
    const addBtnEl = document.querySelector(".add")
    const subBtnEl = document.querySelector(".sub")

    // 2.定义一个变量记录数据
    let counter = 100
    counterEl.textContent = counter

    // 2.监听按钮的点击
    addBtnEl.onclick = function() {
      counter++
      counterEl.textContent = counter
    }
    subBtnEl.onclick = function() {
      counter--
      counterEl.textContent = counter
    }

  </script>

</body>
</html>